Secure Inter-Domain Routing (SIDR) G. Huston
Internet-Draft G. Michaelson
Intended status: Informational APNIC
Expires: April 9, 2009 October 6, 2008
Validation of Route Origination in BGP using the Resource CertificatePKIdraft-ietf-sidr-roa-validation-01.txt
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Abstract
This document defines an application of the Resource Public Key
Infrastructure to validate the origination of routes advertised in
the Border Gateway Protocol. The proposed application is intended to
fit within the requirements for adding security to inter-domain
routing, including the ability to support incremental and piecemeal
deployment, and does not require any changes to the specification of
BGP.
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Internet-Draft Route Validation October 20081. Introduction
This document defines an application of the Resource Public Key
Infrastructure (RPKI) to validate the origination of routes
advertised in the Border Gateway Protocol (BGP) [RFC4271].
The RPKI is based on Resource Certificates. Resource Certificates
are X.509 certificates that conform to the PKIX profile [RFC5280],
and to the extensions for IP addresses and AS identifiers [RFC3779].
A Resource Certificate describes an action by an issuer that binds a
list of IP address blocks and Autonomous System (AS) numbers to the
Subject of a certificate, identified by the unique association of the
Subject's private key with the public key contained in the Resource
Certificate. The PKI is structured such that each current Resource
Certificate matches a current resource allocation or assignment.
This is described in [I-D.ietf-sidr-arch].
Route Origin Authorizations (ROAs) are digitally signed objects that
bind an address to an AS number, signed by the address holder. A ROA
provides a means of verifying that an IP address block holder has
authorized an AS to originate route objects in the inter-domain
routing environment for that address block. ROAs are described in
[I-D.ietf-sidr-roa-format].
Bogon Origin Attestations (BOAs) are digitally signed objects that
describe a collection of address prefixes and AS numbers that are not
authorised by the right-of-use holder to be advertised in the inter-
domain routing system [I-D.ietf-sidr-boa].
This document describes how ROA and BOA validation outcomes can be
used in the BGP route selection process, and how the proposed
application of ROAs and BOAs are intended to fit within the
requirements for adding security to inter-domain routing
[ID.ietf-rpsec-bgpsecrec], including the ability to support
incremental and piecemeal deployment. This proposed application does
not require any changes to the specification of BGP protocol
elements. The application may be used as part of BGP's local route
selection algorithm [RFC4271].
2. Validation Outcomes of a BGP Route Object
A BGP Route Object is an address prefix and a set of attributes. In
terms of ROA and BOA validation the prefix value and the origin AS
are used in the validation operation.
If the route object is an aggregate and the AS Path contains an AS
Set, then the origin AS is considered to be the AS described as the
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Internet-Draft Route Validation October 2008
AGGREGATOR [RFC4271] of the route object.
ROA validation is described in [I-D.ietf-sidr-roa-format], and the
outcome of the validation operation is that the ROA is valid in the
context of the RPKI, or validation has failed.
BOA validation is described in [I-D.ietf-sidr-boa], and the outcome
of the validation operation is that the BOA is valid in the context
of the RPKI, or validation has failed.
There appears to be two means of matching a route object to a ROA:
decoupled and linked.
2.1. Decoupled Validation
The decoupled approach is where the ROAs are managed and distributed
independently of the operation of the routing protocol and a local
BGP speaker has access to a local cache of the complete set of ROAs
and the RPKI data set when performing a validation operation.
In this case the BGP route object does not refer to a specific ROA.
The relying party to match a route object to one or more candidate
valid ROAs and BOAs in order to determine the appropriate local
actions to perform on the route object.
The relying party selects the set of ROAs where the address prefix in
the route object either exactly matches an ROAIPAddress (matching
both the address prefix value and the prefix length), or where the
route object spans a block of addresses that is included in the span
described by the ROA's address prefix value and length and where the
route object's prefix length is less than the ROA's prefix length and
greater then or equal to the ROA's corresponding maxLength attribute.
The following outcomes are possible using the defined ROA validation
procedure for each ROA in this set:
Exact Match:
A valid ROA exists, where the address prefix in the route object
exactly matches a prefix listed in the ROA, or the ROA contains a
covering aggregate and the prefix length of the route object is
smaller than or equal to the ROA's associated maxLength attribute,
and the origin AS in the route object matches the origin AS listed
in the ROA.
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Covering Match:
A valid ROA exists, where an address prefix in the ROA is a
covering aggregate of the prefix in the route object, and the
prefix length of the route object is greater than the ROA's
associated maxLength attribute, and the origin AS in the route
object matches the AS listed in the ROA.
Exact Mismatch:
A valid ROA exists where the address prefix in the route object
exactly matches a prefix listed in the ROA, or the ROA contains a
covering aggregate and the prefix length of the route object is
smaller than or equal to the ROA's associated maxLength attribute,
and the origin AS of the route object does not match the AS listed
in the ROA.
Covering Mismatch:
A valid ROA exists where an address prefix in the ROA is a
covering aggregate of the prefix in the route object, the prefix
length of the route object is greater than the ROA's associated
maxLength attribute, and the origin AS of the route object does
not match the AS listed in the ROA.
No ROA:
There are no Exact Matches, Covering Matches, no Exact Mismatches
or Covering Mismatches in the RPKI repository.
The ROA to be used for the validation function is selected from the
set of ROAs in the order given above. In other words an Exact Match
is preferred over a Covering Match, which, in turn, is preferred over
an Exact Mismatch which is preferred over a Covering Mismatch.
The set of BOAs that are used for the validation function are
composed of the set of valid BOAs where the origin AS of the route
object matches an AS described in a BOA, or where an address prefix
in a valid BOA that is an exact match or a covering aggregate of the
route object. In the case that the validation outcome using ROAs is
one of Exact Mismatch, Covering Mismatch or No ROA, then the
validation outcome of the BOA changes the overall validation result
to "Bogon".
Bogon:
A valid BOA exists where an address prefix in the BOA is a an
exact match for the prefix in the route object, or is a covering
aggregate of the prefix in the route object, or an AS in the BOA
matches the originating AS in the BOA. In addition, there is no
valid ROA that is an Exact Match or a Covering Match with the
route object.
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Internet-Draft Route Validation October 20082.2. Linked Validation
The linked approach requires the route object to reference a ROA
either by inclusion of the ROA as an attribute of the route object,
or inclusion of a identity field in an attribute of the route object
as a means of identifying a particular ROA.
If the ROA can be located is valid within the context of the RPKI
then the route object can be compared against the ROA, as per the
previous section, giving one of five possible results: Exact Match,
Covering Match, Exact Mismatch, Covering Mismatch, and No Match,
which is defined as:
No Match:
The valid ROA does not comtain any address prefix that exactly
matches the address prefix in the route object, or is a covering
aggregate of the address prefix in the route object.
In the case of a Mismatch or a No Match condition, the relying party
should check for the presence of valid BOAs where the origin AS of
the route object matches an AS described in a BOA, or where an
address prefix in a valid BOA that is an exact match or a covering
aggregate of the route object. If a valid BOA can be found that
matches either of these conditions that the overall route object
validation of a route object with a linked ROA is changed to "Bogon".
3. Applying Validation Outcomes to BGP Route Selection
Within the framework of the abstract model of BGP operation, a
received prefix announcement from a peer is compared to all
announcements for this prefix received from other peers and a route
selection procedure is used to select the "best" route object from
this candidate set which is then used locally by placing it in the
loc-RIB, and is announced to peers as the local "best" route.
It is proposed here that the validation outcome be used as part of
the determination of the local degree of preference as defined in
section 9.1.1 of the BGP specification [RFC4271].
In the case of partial deployment of ROAs there are a very limited
set of circumstances where the outcome of ROA validation can be used
as grounds to reject all consideration of the route object as an
invalid advertisement. While the presence of a valid ROA that
matches the advertisement is a strong indication that an
advertisement matches the authority provided by the prefix holder to
advertise the prefix into the routing system, the absence of a ROA or
the invalidity of a covering ROA does not provide a conclusive
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Internet-Draft Route Validation October 2008
indication that the advertisement has been undertaken without the
address holder's permission, unless the object is described in a BOA.
In the case of a partial deployment scenario of RPKI route
attestation objects, where some address prefixes and AS numbers are
described in ROAs or BOAs and others are not, then the relative
ranking of validation outcomes from the highest (most preferred) to
the lowest (least preferred) degree of preference are proposed to be
as specified int he following list. The exact values to apply to a
Local Preference setting are left as a matter of local policy and
local configuration.
1. Exact Match
The prefix has been allocated and is routeable, and that the
prefix right-of-use holder has authorized the originating AS to
originate precisely this announcement.
2. Covering Match
This is slightly less preferred because it is possible that the
address holder of the aggregate has allocated the prefix in
question to a different party. It is also possible that the
originating AS is using more specific advertisements as part of a
traffic engineering scenario.
3. No ROA
In the case of partial deployment of ROAs, the absence of
validation credentials is a neutral outcome, in that there is no
grounds to increase or decrease the relative degree of preference
for the route object.
4. Covering Mismatch
A Covering Mismatch is considered to be less preferable than a
neutral position in that the address holder of a covering
aggregate has indicated an originating AS that is not the
originating AS of this announcement. On the other hand it may be
the case that this prefix has been validly allocated to another
party who has not generated a ROA for this prefix even through
the announcement is valid.
5. Exact Mismatch
Here the exact match prefix holder has validly provided an
authority for origination by an AS that is not the AS that is
originating this announcement. This would appear to be a bogus
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Internet-Draft Route Validation October 2008
announcement by inference.
6. No Match
Here the route object has referenced a ROA that is not valid, or
does not include an address prefix that matcehs the route object,
or the referenced ROA could not be located. This could be an
attempt to create a false route object and use an invalid ROA.
7. Bogon
Here the right-of-use holder of the AS or address prefix has
explicitly tagged the address prefix or the AS as a "bogon".
This implies that the announcement has been made without the
appropriate authority, and the local preference of the route
object should be ranked at a level commensurate with rejecting
the route object.
In the case of comprehensive deployment of RPKI route attestion
objects the absence of a specific ROA origination authority for the
route object should render it as an unusable for routing. In this
case the local preference setting for the route object is as follows:
1. Exact Match
The prefix has been allocated and is routeable, and that the
prefix right-of-use holder has authorized the originating AS to
originate precisely this announcement.
2. Covering Match, No ROA, Covering Mismatch, Exact Mismatch, No
Match
The local preference of the route object should be ranked at a
level of least preferred, due to the constraints noted in the
following section.
3. Bogon
Here the right-of-use holder of the AS or address prefix has
explicitly tagged the address prefix or the AS as a "bogon".
This implies that the announcement has been made without the
appropriate authority, and the local preference of the route
object should be ranked at a level commensurate with rejecting
the route object.
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Internet-Draft Route Validation October 20083.1. Validation Outcomes and Rejection of BGP Route Objects
In the case of comprehensive deployment of ROAs, the use of a
validation outcome other than an Exact Match as sufficient grounds to
reject a route object should be undertaken with care.
The consideration here is one of potential circularity of dependence.
If the authoritative publication point of the repository of ROAs or
any certificates used in relation to an address prefix is stored at a
location that lies within the address prefix described in a ROA, then
the repository can only be accessed once a route for the prefix has
been accepted by the local routing domain. It is also noted that the
propagation time of RPKI objects may be different to the propagation
time of route objects in BGP, and that route objects may be received
before the relying party's local repository cache picks up the
associated ROAs and recognises them as valid within the RPKI.
For these reasons it is proposed that, even in the case of
comprehensive deployment of ROAs, a missing ROA or a mismatch should
not be considered as sufficient grounds to reject a route
advertisement outright. Alternate approaches may involve the use of
a local timer to accept the route for an interim period of time until
there is an acceptable level of assurance that all reasonable efforts
to local a valid ROA have been undertaken.
4. Further Considerations
This document provides a description of how ROAs and BOAs could be
used by a BGP speaker.
It is noted that the proposed procedure requires no changes to the
operation of BGP.
It is also noted that the decoupled and linked approach are not
mutually exclusive, and the same procedure can be applied to route
objects that contain an explicit pointer to the associated ROA and
route objects where the local BGP speaker has to create a set of
candidate ROAs that could be applied to a route object. However,
there are a number of considerations about this approach to
origination validation that are not specified here.
These considerations include:
o It is not specified when validation of an advertised prefix should
be performed by a BGP speaker. Is is considered to be a matter of
local policy whether it is considered to be strictly necessary to
perform validation at a point prior to loading the object into the
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Adj-RIB-In structure, or once the object has been loaded into Adj-
RIB-In, or at a later time that is determined by a local
configuration setting. It is also not specified whether
origination validation should be performed each time a route
object is updated by a peer even when the origin AS has not
altered.
o The lifetime of a validation outcome is not specified here. This
specifically refers to the time period during which the original
validation outcome can be still applied, and the time when the
routing object be revalidated. It is a matter of local policy
setting as to whether a validation outcome be regarded as valid
until the route object is withdrawn or further updated, or whether
validation of a route object should occur at more frequent
intervals?
o It is a matter of local policy as to whther there are
circumstances that would allow a route object to be removed from
further consideration in route selection upon a validation
failure, similar to the actions of Route Flap Damping.
o It is a matter of local configuration as to whther ROA validation
is performed on a per-AS basis rather than a per-BGP speaker, and
the appropriate BGP mechanisms to support such a per-AS iBGP route
validation service are not considered here.
5. Security Considerations
This approach to orgination validation does not allow for
'deterministic' validation in terms of the ability of a BGP speker to
accept or reject an advertised route object outright, given that
there remains some issues of potential circularity of dependence and
time lags between the propagation of information in the routing
system and propagation of information in the RPKI.
There are also issues of the most appropirate interpretation of
outcomes where validation of the authenticity of the route object has
not been possible in the context of partial adoption of the RPKI,
where the absense of validation information does not necessarily
constitute sufficient grounds to interpret the route object as an
invalidly originated object.
The consequence of these considerations is that while the use of ROAs
can increase the confidence in the validity of origination of route
objects that match a valid ROA, ROAs cannot perform the opposite,
namely the rejection of route objects that cannot be validated by
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